I have an 8400GS that has an S-Video out. If I connect the card to my TV via an S-video cable without connecting the monitor at all, I can watch Linux (Arch-i686) boot on the TV. Problem for me is when I start X. The TV goes blank. If I plug in a CRT monitor, X is being displayed there rather than on the TV. I ran nvidia-xconfig to generate my /etc/X11/xorg.conf and I even tried messing around with it a bit (adding the tvout options) but still have nothing on the TV.

If I have the TV connected and I also connect a CRT monitor, I can get into X just fine. The nvidia-settings program doesn't detect my TV at all. Arch uses xorg-server v1.7.2 if that matters. I'm using the nvidia driver v190.42.

EDIT: I should say that if I switch out the 8400GS (PCI) for an older 6200 (AGP) everything just works without any modifications to the /etc/X11/xorg.conf at all!